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NANO EXPRESS Open Access
Performance evaluation on an air-cooled heat
exchanger for alumina nanofluid under laminar flow
Tun-Ping Teng
1*
, Yi-Hsuan Hung
1
, Tun-Chien Teng
2
and Jyun-Hong Chen
1
Abstract
This study analyzes the characteristics of alumina (Al
2
O
3
)/water nanofluid to determine the feasibility of its
application in an air-cooled heat exchanger for heat dissipation for PEMFC or electronic chip cooling. The
experimental sample was Al
2
O
3
/water nanofluid produced by the direct synthesis method at three different
concentrations (0.5, 1.0, and 1.5 wt.%). The experiments in this study measured the thermal conductivity and
viscosity of nanofluid with weight fractions and sample temperatures (20-60°C), and then used the nanofluid in an
actual air-cooled heat exchanger to assess its heat exchange capacity and pressure drop under laminar flow.
Experimental results show that the nanofluid has a higher heat exchange capacity than water, and a higher
concentration of nanoparticles provides an even better ratio of the heat exchange. The maximum enhanced ratio
of heat exchange and pressure drop for all the experimental parameters in this study was about 39% and 5.6%,
respectively. In addition to nanoparticle concentration, the temperature and mass flow rates of the working fluid
can affect the enhanced ratio of heat exchange and pressure drop of nanofluid. The cross-section aspect ratio of

tube in the heat exchanger is another important factor to be taken into consideration.
Keywords: alumina (Al
2
O
3
), heat exchange capacity, laminar flow, nanofluid, pressure drop
Introduction
As technology and energy products require higher stan-
dard s of function and performance, the problem of heat
dissipation is becoming a significant issue in many
appliances. Using a working fluid with high heat transfer
performance is a topic worthy of research, as it may
solve this problem without costly changes in the struc-
ture of the equipment. Many researchers have recently
investigated the issue of nanofluid thermal properties.
Many studies show that nanofluids can enhance heat
conduction performance due to their higher thermal
conductivity than base fluids [1-6]. However, heat con-
vection characteristics must also be considered in practi-
cal heat exchange applications. Many researchers have
focused on heat transfer properties of convection for a
single pipe with the different structures, and investigated
the parameters of nanoparticles added, pipe cross-sec-
tion structure, materials and concentration of nanofluid,
flow conditions, and other factors [7-14].
Palm et al. [15] reported that the water/Al
2
O
3
nano-

fluid with concentration of 4 vol.% enhanced the average
wall heat transfer coefficient by 25% compared to base
liquid in 2006. Furthermore, the average heat transfer
coefficient increased with an increase in wall heat flux
due to a decrease in the wall shear stress.
Nguyen et al. [16] used Al
2
O
3
nanoflui d with different
nanoparticle sizes (36 and 47 nm) in an electronic liquid
cooling system. The heat convective coefficient was
enhanced by a maximum of 40% at an added particle
concentration of 6.8 vol.%. The heat convective coeffi-
cient of t he added n anoparticle size 36 nm was higher
than that of 47 nm at the same concentration. These
results show that nanofluid improve the heat transfer
performance for electronic liquid cooling system, and
smaller nanoparticles added to the based liquid more
effectively enhanced the heat convective coefficient.
Chein and Chuang [17] applied CuO/water nanofluid to
a microchannel heat sink (MCHS) and found that a
nanoparticle concentration of 0.2 to 0.4 vol.% enhanced
the cooling performance of CuO/water nanofluid. Their
experimental results show that the CuO/water nanofluid
had low thermal resistance at lower flow rate (10 and 15
* Correspondence:
1
Department of Industrial Education, National Taiwan Normal University, No.
162, Section 1, He-ping East Road, Da-an District, Taipei City 10610, Taiwan,

Republic of China
Full list of author information is available at the end of the article
Teng et al. Nanoscale Research Letters 2011, 6:488
/>© 2011 Teng et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution
License (http://creativecommons .org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
ml/min), and higher resistance at higher flow rate (20
ml/min). These results indicate that the flow rate is a
very important fac tor to affect the heat convective per-
formance of a nanofluid.
Kulkarni et al. [18] studied the specific heat of Al
2
O
3
/
ethylene glycol and water (EG/W) nanofluid and its
effect on the cogeneration efficiency of a 45-kW diesel
electric generator (DEG) in 20 08. Their experimental
results show that applying nanofluid reduced cogenera-
tion efficiency due to a decrease in the specific heat of
the nanofluid. Further, the efficiency of waste heat
recovery in the heat exchanger increased due to the
higher convective heat transfer coefficient of the
nanofluid.
Pantzali et al. [19] adopted a 4 vol.% CuO nanofluid
to investigate the effects of using nanofluid in a minia-
ture plate heat exchanger with a modulated surface
through both experimental and numerical calculations.
Their results demonstrate that the CuO nanofluid
enhanced the heat transfer rate and total heat transfer

coe fficient, and suggested that the required flow rate of
nanofluid was lower than that of water to keep lower
pressure drop. Jung et al. [20] studied the convective
heat transfer coefficient and friction factor of Al
2
O
3
-
water/ethylene glycol (50:50) nanofluid with different
concentrations (0.6, 1.2, 1.8 vol.%) in rectangular micro-
channels. They also measured the Al
2
O
3
nanoparticles
size of 170 nm in nanofluid using light scattering equip-
ment. The convective heat transfer coefficient of the
Al
2
O
3
nanofluid at 1.8 vol.% increased 32% compared
to the base liquid without a major friction loss in a
laminar flow regime (5 < Re < 300). The Nusselt num-
ber increased as the Reynolds number increased in a
laminar flow regime. Nnanna et al. [21] adopted Al
2
O
3
/

water nanofluid for heat dissipation in the heat exchan-
ger of a thermoelectric module. The nanoparticle size
and concentration of adde d nanoparticles in that study
were 27 nm and 2 vol.%, respectively. The average ther-
mal conta ct resistance was 0.18°C/W and 0.12°C/W for
the deionized water and nanofluid, respectively.
Duangthongsuk and Wongwises [22] reported an
experimental study on forced convective heat transfer
under varied heat flux boundary conditions and pres-
sure drop characteristics of a nanofluid with 0.2 vol.%
TiO
2
nanoparticles (d
p
= 21 nm) flowing in a horizontal
double-tube counter flow heat exchanger under turbu-
lent flow regimes. Their results show that the convec-
tive heat transfer coefficient of nanoflu id is
approximately 6% to 11% higher than that of the base
liquid. The heat transfer coe fficient of the nanofluid
increased as the mass flow rate of the water and nano-
fluid increased.
Abu-Nada et al. [23] used an efficient f inite-volume
method to study the heat transfer characteristics of
natural convection for CuO/EG/water nanofluid in a dif-
ferentially heated enclosure. They presented various
results for the streamline and isotherm contours and the
local and average Nusselt numbers for a wide range of
Rayleigh numbers (Ra = 10
3

to approximately 10
5
),
nanoparticle concentrations (0 <j < 6 vol.%), and enclo-
sure aspect ratios (1/2 ≦ A ≦ 2). Their results show that
the enclosure aspect ratio had significant effects on the
behavior of the average Nusselt number, which
decreased as the enclosure aspect ratio increased. Ho et
al. [24] investigated the f orced convective cooling per-
formance of a c opper MCHS with Al
2
O
3
/water nano-
fluid as the coolant under laminar flow conditions (Re =
226 to approximately 1,676). Their results show that the
dynamic viscosity and friction factor increased due to
dispersing the alumina nanoparticles in water. The
MCHS with Al
2
O
3
/water nanofluid also had higher
average heat transfer coefficient, lower thermal resis-
tance, and lower wall te mperature at high pumping
power. Feng and Kleinstreuer [25] presented numerical
simulatio ns for heat transfer between parallel disks with
an Al
2
O

3
/water nanofluid flow. Their results indicate
that the nanofluid had smoother mixture flow fields and
temperature distributions. The Nusselt number
increased with higher nanoparticle volume fraction,
smal ler nanoparticle size, reduced disk spacing, and l ar-
ger inlet Reynolds number under a realistic thermal
load. They also proposed the correlation of critical radial
distance and minimization of total entropy g eneration
analysis. Jwo et al. [26] adopted Al
2
O
3
/water nanofluid
for heat dissipation experiments in a multi-channel heat
exchanger (MCHE) to simulate its application to elec-
tronic chip cooling system. Results show that the overall
heat transfer coefficient ratio was higher at higher nano-
particle concentrations. In addition, when the input
temperature of nanofluid flowing into MCHE was lower,
the mass flow rate had a greater effect on the overall
heat transfer coefficient ratio than concentration. Fara-
jollahi et al. [ 27] performed an experimental analysis to
study heat transfer of nanofluid in a shell and tube heat
exchanger. They used nanofluid Al
2
O
3
/water and TiO
2

/
water nanofluid under turbulent flow conditions to
investigate the effects of the Peclet number, volume
concentration of suspended particles, and particle type
on heat transfer characteristics. Their results indicate
that the addition of nanoparticles to the base fluid
enhances heat transfer performance. Notice that heat
transfer characteristics of nanofluid increased signifi-
cantly with the Peclet number. TiO
2
/water and Al
2
O
3
/
water nanofluid exhibited better heat transfer behavior
at lower and higher volume concentrations, respectively.
The experimental results above are also in agreement
with the predicted values of available correlation at the
lower volume fractions of the nanoparticle.
Teng et al. Nanoscale Research Letters 2011, 6:488
/>Page 2 of 11
Firouzfar et al. [28] recently used a methanol/Ag
nanofluid to fill a thermosyphon heat exchanger and
compared its effectiveness and energy saving with that
of pure methanol. Their experimental results show that
methanol/Ag nanofluid achieved an energy savings of
approximately 8.8-31.5% for cooling and 18-100% for
reheating the supply air stream in an air conditioning
system, respectively. Zamzamian et al. [29] investigated

the effects of forced convective heat transfer coefficient
with Al
2
O
3
/EG and CuO/EG nanofluid in double pipe
and plate heat exchangers. Their results indicate that
increasing the nanoparticle concentrat ion and tempera-
ture could enhance the convective heat transfer coeffi-
cient of nanofluid, leading to a 2% to 50% en hancement
in convective heat transfer coefficient of the nanofluid.
The literature review above clearly show s that using
nanofluid can effectively improve the heat convective
performance, but will also increase the pip eline pressure
drop and pumping energy. Using nanofluid w ith a high
heat convective performance for heat exchange can help
reduce the volume of the heat exchanger. In addition,
using nanofluid with higher heat transfer performance
instead of the traditional working fluid for cooling can
reduce the demand and cost of cooling system modifica-
tions. Most of the nanofluids used in previous studies
were used in single pipe heat transfer, microchannel
heat sinks, plate heat exchangers, double-tube heat
exchangers, or heated enclosures, and seldom used in
air-cooled heat exchangers. Since the ultimate goal of
radiators is to discharge heat into the atmosphere, and
the air-cooled heat exchanger is widely used in autom o-
tive, air conditioning, proton exchange membrane fuel
cell (PEMFC) and electronic chip cooling, and is there-
fore a worthy research directio n. This study uses a two-

step synthesis method to make Al
2
O
3
/water nanofluid,
which can be used as coolant in an air-cooled heat
exchanger to heat dissipation . Identifying the differences
in nanofluid weight fractions, mass flow rates, and tem-
perature effects on heat exchange performance and
pressure drop of the air-cooled heat exchanger makes it
possible to evaluate the feasibility of applying Al
2
O
3
/
water nanofluid to PEMFC heat dissipation or electronic
chip cooling in the future.
Calculation for heat exchange and flow conditions
This section evalua tes the heat exchange capacity of the
working fluid for an air-cooled heat exchanger based on
the measured inlet and outlet temperature difference
(T
i
-T
o
) for different mass flow rates (
˙
m
f
) and specific

heat (c
p, f
). The heat exchange capacity (
˙
Q
e
x
) of the heat
exchanger can be written as follows:
˙
Q
ex
=
˙
m
f
c
p
,
f
(
T
i
− T
o
)
.
(1)
Under the condition of actual application, the cross-
section of pipe is not circular, so modification is needed.

The characteristic length of a non-circular cross-section
is called hydraulic radius (R), and can be expressed as
R =
A
WP
,
(2)
where A is the area of cross-section, and WP is the
wetter perimeter (rectangle side lengths equal to a and
b, then WP = 2a +2b).
The Reynolds number (Re) of the flow in the non-cir-
cular cross-section pipe can be expressed as
Re
nc
=
ρ
f
v
m
4
R
μ
f
.
(3)
According to the concept of solid-liquid mixture, the
density (r
nf
) and specific heat (c
p,nf

)oftheAl
2
O
3
/water
nanofluid is given by Equations 4 and 5, with volume
fraction (j), bulk fluid density (r
bf
), nanoparticle density
(r
p
), bulk fluid specific heat (c
p, bf
), and nanoparticle
specific heat (c
p, p
) [ 2,30,31]:
ρ
nf
=(1− φ)ρ
bf
+ φρ
p
,
(4)
c
p,
nf
=(1− φ)c
p,

bf
+ φc
p,
p
.
(5)
The volume fraction (j)oftheAl
2
O
3
/water nanofluid
is given by Equation 6, with bulk fluid weight (W
bf
),
nanoparticle weight (W
p
) and nanofluid weight (W
nf
):
φ =(W
p
/ρ
p
)/(W
nf
/ρ
nf
)=ω

ρ

nf
/ρ
p

.
(6)
Equation 6 can be used to convert the weight fraction
to volume fraction to calculat e the density and specific
heat of nanofluid by Equations 4 and 5. The density and
specific heat of nanof luid and experimental data can
then be used to calculate the Reynolds number and heat
exchange capacity for the nanofluid.
Preparation of sample and experimental design
Preparation of alumina nanofluid
The base liquid was prepared by adding 0.2 wt.% of
cationic dispersant (water-soluble chitosan) to distilled
water as a dispersant to obtain good suspension for
nanofl uid. The Al
2
O
3
/water nanofluid produced by two-
step synthesis method was then used as the experimen-
tal sample, and homogenization, electromagnetic agita-
tion, and ultrasonic vibration were alternately used to
disperse the Al
2
O
3
nanoparticles into three weight frac-

tions (0.5, 1.0, 1.5 wt.%) in the base liquid. The r eason
for using a lower concentration of nanofluid was to
avoid blocking pipes and an overly high pressure drop
caused by the sedimentation of nanoparticles and
Teng et al. Nanoscale Research Letters 2011, 6:488
/>Page 3 of 11
increased viscosity from a high concentration of nano-
fluid. The Al
2
O
3
/water nanofluid used in this study con-
tains commercial nanoparticles (Al-13P, Yong-Zhen
Technomaterial, Taipei, Taiwan). The real density of
Al
2
O
3
nanoparticles is approximately 3,880 kg/m
3
,
which can be converted to be weight fraction and
volume fraction by Equation 6.
Figures 1 and 2 respectively show field emission scan-
ning electron microscope (FE-SEM, S-4800, Hitachi,
Tokyo, Japan) and transmission electron microscope
(TEM, H-7100, Hitachi) photographs of Al
2
O
3

nanopar-
ticles. These figures show that the nanoparticles exhibit
an aggregate phenomenon, and the primary particle size
is about 20 nm. The crystalline phase of Al
2
O
3
nanopar-
ticle was determined by X-ray Diffraction (XRD, APEX
II, Kappa CCD, Monrovia, CA, USA). All peaks were
measur ed by XRD and compared with those of the joint
committee on powder diffraction standards data
(PCPDFWIN 2.4, JCPDS-ICDD, Newtown S quare, PA,
USA) [32] (Figure 3). This figure confirms that the
material used in this study was g-alumina. All the com-
pleted experimental samples were allowed to remain sta-
tic for 7 days to confirm suspension performance.
Spectrometer analysis confirmed that the concentration
of Al
2
O
3
/water nanofluid changed less than 5%.
Experimental procedure and design
This study investigates whether the Al
2
O
3
/water nano-
fluid can be used for PEMFC heat dissipation or electro-

nic chip cooling in the future. Thus, the temperature of
the test samples was set at 30°C to approximately 60°C
to simulate the most common cooling temperature
range in electronic cooling and PEMFC heat dissipation.
Firstly, in the thermal conductivity and viscosity
experime nts, a thermostatic bath (D-620, DengYng, Tai-
pei, Taiwan) was stabilized the temperature of the sam-
ple until it reached the expected temperature (20°C to
approximately 60 ± 0.5°C). A thermal properties analy-
zer (KD-2 Pro, Decagon Devices, Inc., Pullman, WA,
USA) and rheometer (DVIII+, Brookfield, Middleboro,
MA, USA) were then used to measure the thermal con-
ductivity and viscosity in the nanofluid at various weig ht
fractions and sample temperatures. The suspended par-
ticlesizeofAl
2
O
3
/water nanofluid was then measured
using a dynamic light scattering (DLS) size/zeta poten-
tial analyzer (SZ-100, HORIBA, Kyoto, Japan) to deter-
mine clustering and suspension performance.
Figure 1 FE-SEM images of Al
2
O
3
nanoparticles.
Figure 2 TEM images of Al
2
O

3
nanoparticles.
Figure 3 XRD patterns of Al
2
O
3
nanoparticles.
Teng et al. Nanoscale Research Letters 2011, 6:488
/>Page 4 of 11
The heat exchange and pressure drop experiments in
this study used a heated tank to simulate the heating
source, and evaluated the cooling performance of nano-
fluid using air-cooled heat exchangers under the condi-
tions for different concentrations, temperatures,
nanofluid mass flow rates. Figure 4 shows the experi-
mental setup for the heat exchange capacity experiment.
Figure 5 shows the construction of the rectangular tube
in the air-cooled heat exchanger used in this study.
After 2,200 ml of test samples were poured into a 2.5-
liter acrylic tank and the sample temperature was con-
trolled by a PID temperature controller (TTM-J4,
TOHO, Japan) with SSR (SSR-40DA, Manax, Taiwan)
and heater (300 W), the nanofluid was pumped to an
air-cooled heat exchanger for circulation.
The heat exchange capacity of the liquid side was
calculated based on measurements of the temperature
difference and flow rate between the inlet and outlet
of the liquid of heat exchanger. The air-cooled heat
exchanger was made of aluminum, and its structure
was of finned-tube type assembled with 11 rectangu-

lar tubes at 118 × 17.3 × 1.9 mm (L × W × H)each.
The effective internal cross-sectional area was 2.17 ×
10
-5
m
2
. The pipe w as covered by thermal insulation
material at the thickness of 1.5 cm to reduce the
influence of heat dissipation from other components.
The mass flow rate of liquid side was controlled by
the input voltage (GPC-6030D, GWINSTEK, Taipei,
Taiwan) of circulating pump (MCP-655, Swifttech,
USA). This experiment used a temperature controller
to stabilize the temperature of the sample until it
reached the expected temperature (30°C, 40°C, 50°C,
and 60 ± 0.5°C). An environmental control system
maintained the temperature and relative humidity at
25 ± 1°C and 60 ± 5% to ensure the constant environ-
mental conditions at the air side of heat exchanger,
and kept the air side conditions of each experiment
the same under fixed air flow rate. A multifunction
meter (Testo-400, Testo, Lenzkirch, Germany) moni-
tored the environmental conditions to ensure the sta-
bility of the experiment. A data logger (TRM-20,
TOHO, Japan), a pressure transducer (JPT-131LJ,
Jetec, Taichung, Taiwan) and a flow meter (NF05,
Aichi Tokei, N agoya, Japan) were also employed to
measure the temperature, pressure, and flow rate to
coordinate with the relevant equations to calculate
theheatexchangecapacity.

Data and uncertainty analysis
The results of heat exchange capacity and pressu re drop
obtained with the distilled water were used as baseline
values (D
bf
) to a llow easy comparison of experimental
data after c hanging the Al
2
O
3
/water nanofluid (D
nf
). In
other words, the experimental data obtained with the
Al
2
O
3
/water nanofluid was compared with baseline
values. The differences between the before-and-after
changes by the Al
2
O
3
/water nanofluid were presented as
proportions (ER), and can be calculated as follows:
ER =

(
D

nf
− D
bf
)
/D
bf

× 100%.
(7)
Figure 4 Experimental setup for heat exchange capacity
measurement.
Figure 5 Construction of the rectangular tube in air-c ooled
heat exchanger.
Teng et al. Nanoscale Research Letters 2011, 6:488
/>Page 5 of 11
The uncertainty of the experimental results was deter-
mined based on the measurement deviation of the para-
meters, including thermal conductivity, viscosity, flow
rate, input voltag e, weight, and temperature. The ther-
mal conductivity experiment calculated the thermal con-
ductivity based on readings of the thermal property
analyzer (k). The weight (W) of nanoparticles was mea-
sured by a pre cise electric b alance (XT-620 M, Precisa
Dietikon, Switzerland). The temperature of the isother-
mal bath (T) was measured by resistance temperature
detector (RTD, pt-100).
u
m,k
=



k/k

2
+

W/W

2
+

T/T

2
(8)
The precision of the thermal property analyzer is ±
5%. The accuracy of the precise electric balance is ±
0.01 g. The precision of the RTD is ± 0.5°C Hence, the
uncertainty of the thermal conductivity experiment was
calculated to be less than ± 5.6%.
The viscosity experiment calculated the viscosity based
on readings of the rheometer (μ). The weight (W)of
nanoparticles was measured by a precise electric bal-
ance. The temperature of isothermal bath (T)wasmea-
sured by resistance temperature detector (RTD, pt-100).
u
m,μ
=



μ
t
/μ
t

2
+(W/W)
2
+

T/T

2
(9)
The precision of the rheometer is ± 1%. The accuracy
of the precise electric balance is ± 0.01 g. The precision
of the RTD is ± 0.5°C Hence, the uncertainty of the
viscosity experiment was calculated to be less than ±
2.7%.
The Reynolds number (Re) experiment on nano fluid
measured the flow veloc ity rate (v
m
) using a flow meter
and cross-sectional area. The viscosity was determined
based on readings of the rheometer (μ). The weight (W)
of nanoparticles was measured by a precise electric bal-
ance, and the temperature was determined using ther-
mocouples (T; T-type). Ignoring the calculation
deviations generated by Equations 4, 5, and 6 and tube
size, the unc ertainty of these experimental results can

be expressed as follows:
u
m,Re
=

(v
m
/v
m
)
2
+(μ/μ)
2
+

W/W

2
+

T/T

2
.
(10)
The accuracy of the flow meter is ± 2.0%. The preci-
sion of the rheometer is ± 1%. The accuracy of the pre-
cise electric balance is ± 0.01 g. The accuracy of the
thermocouple is ± 0.5°C. Therefore, the uncertainty of
the Re experiment was calculated to be less than ± 3.4%.

The pressure drop experiment on nanofluid measured
the mass flow rate (
·
m
) using a flow meter and density
of liquid. The pressure drop (dP) of the liquid was
measured by a pressure transducer. The weight (W)of
nanoparticles was measured by a precise electric bal-
ance, and temperature was determined using thermo-
couples (T-type, T). Ignoring the calculation deviations
generated by Equations 4, 5. and 6, the uncertainty of
these experimental results can be expressed as follows:
u
m,dp
=

(
·
m
/
·
m
)
2
+

dP/dP

2
+


W/W

2
+

T/T

2
.
(11)
The accuracy of the flow meter is ± 2.0%. The accu-
racy of the pressure transducer is ± 0.5%. The accuracy
of the precise electric balance is ± 0.01 g. The accuracy
of the thermocouple is ± 0.5°C. Therefore, the uncer-
tainty of the pressure drop experiment was calculated to
be less than ± 3.3%.
The heat exchange capacity experiment o n nanofluid
measured the mass flow rates (
·
m
)usingaflowmeter
and density of liquid. The w eight (W) of nanoparticles
wasmeasuredbyapreciseelectricbalance,andtem-
perature was determined using thermocouples (T-type,
T). Ignoring the calculation deviations generated by
Equations 1, 4, 5, and 6), the uncertainty of experimen-
tal results can be expressed as follows:
u
m,he

=

(
·
m
/
·
m
)
2
+

W/W

2
+

T/T

2
.
(12)
The accuracy of the flow meter is ± 2.0%. The accu-
racy of the precise electric balance is ± 0.01 g. The
accuracy of the thermocouple is ± 0.5°C. Therefore, the
uncertainty of the heat exchange capacity experiment
was calculated to be less than ± 3.3%.
Results and discussion
This study uses a dynamic light scattering size/zeta
potential analyzer to determine the average size of the

nanoparticle suspended in base liquid. Figure 6 shows
theparticlesizedistributionoftheAl
2
O
3
nanoparticles
suspended in base liquid. The z-average particle size
and zeta potential is 149.9/33.6 mV, 129.5/41.4 mV, and
135.1/42.1 mV at 0.5 wt.%, 1.0 wt.%, and 1.5 wt.%,
respectively. These distrib utions have a single peak, and
the particle size distribution concentrated between 80 to
approximately 310 nm. The tested particle size from
DLS size/zeta potential analyzer exceeded the particle
size observed by FE-SEM and TEM for the following
two reasons: (1) The particle size analyz er measures the
nanoparticle size based on the principle of dynamic light
scattering,andisthereforeaffectedbytheviscosityand
refractive index of solution. This is because viscosity
and refractive index both affect the mobility of nanopar-
ticles in solution, causing deviations in the measure-
ment. (2) Because the agglomeration of nanoparticles
continues to occur as the nanoparticles are suspended
Teng et al. Nanoscale Research Letters 2011, 6:488
/>Page 6 of 11
in the base liquid, the tested particle size is greater than
theparticlesizeobservedbyFE-SEMandTEM(Figure
1 and 2).
Figure 7 depicts the changes in thermal conductivity
for nanofluid at various temperatures and concentra-
tions over a temperature range of 20°C to 60°C. This

figure reveals that as the temperature increases, the
effect of increasing nanoparticle concentration on the
thermal conductivity ratio is lower than changing the
applied temperature. Increasing both the concentration
and temperature raises the probability that nanoparticle-
liquid collisions will produce a near quasi-convection
phenomenon. Increasin g random collision behavior
helps increase the thermal conductivity of Al
2
O
3
/water
nanofluid. However, some researchers believe that these
factors do not cause a significant increase in thermal
conductivity [33,34]. For a concentration of 0.5 wt.%
and a temperature in the range of 20°C to 60°C, the
thermal conductivity ratio increases by 1.1% to 17.2%.
For concentration of 1.0 wt.%, the thermal conductivity
ratio increases by 1.8% to 19.7%. For a concentratio n of
1.5 wt.%, the thermal conductivity ratio i ncreases by
4.2% to 20.5% compared to water.
Figure 7 also reveals an underestimation between the
Pak and Cho’s model [30] and the current experimental
results. The nanoparticle volume fraction was trans-
formed into the nanoparticl e weight fraction using the
true density of nanoparticles to unify the concentration
of units (Equation 6). Pak and Cho’s model [30] was ori-
ginally obtained with a temperature of 300 K, a particle
size of 13 nm, and a concentration range of 1.34-4.33
vol.%. Because this model does not incorporate changes

of temperature and particle size, it originally obtained at
a higher concentrat ion, and its deviation is a little
higher. However, considering the uncertainty of the
experiment in this study, this deviation is within an
acceptable range under 20°C to 30°C.
Figure 8 depicts the changes in viscosity for Al
2
O
3
/
wat er nanofluid at vari ous temperatures and concentra-
tions. In general, the nanofluid viscosity increases with
increasing nanoparticle loading in the base liquid. For a
concentration of 0.5 wt.% and within a temperature
range of 20°C to 60°C, the viscosity ratio increases by
21.5% to 41.3%. For a concentration of 1.0 wt.%, the
Figure 6 The particle size distribution for the Al
2
O
3
nanoparticles suspended in base liquid.
Figure 7 Thermal conductivity of Al
2
O
3
/water nanoflui d at
various temperatures and concentrations.
Figure 8 Viscosity of Al
2
O

3
/water nanofluid at various
temperatures and concentrations.
Teng et al. Nanoscale Research Letters 2011, 6:488
/>Page 7 of 11
viscosity ratio increases by 32.7% to 47.8%. For a con-
centration of 1.5 wt.%, the v iscosity ratio increases by
38.7% to 56.3%. These results show that the viscosity of
Al
2
O
3
/water nanofluid is much higher than w ater. The
pressure drop of pipeline-related issues must be consid-
ered when the Al
2
O
3
/wate r nanofluid is applied to heat
exchange.
Figure 9 shows the change in Reynolds number (Re)
for Al
2
O
3
/water nanofluid at various temperatures and
concentrations for different mass flow rates. This figure
reveals that at the same mass flow rate, Re increases
with the increasing temperature of nanofluid, but Re
decreases with the increasing concentration of nano-

fluid. The whole experimental range of Re is limited to
the laminar flow range (< < 2000). In gene ral, the visc-
osity (Figure 8) and density (Equation 4) of nanofluid
increases with increasing nanoparticle loading in the
base liquid, and the viscosity ratio of the nanofluid is
greater than the enhanced density ratio of the nanofluid.
At the same mass flow rate, the higher density of the
fluid leads to a lower flow velocity. Thus, the Re of the
nanofluid will be lower than water a t the same mass
flow rate and temperature conditions.
Figures 10, 11, and 12 show the effects of different
concentration, inlet temperature, and mass flow rates of
nanofluid on the enhan ced ratio of heat exchange capa-
city (ER
he
). Results show that nanofluid can enhance the
air-cooled heat exchange capacity ratio under all experi-
mental conditions investigated in this study. This is pri-
marily because the added nanoparticles improved the
heat transfer performance of t he fluid. The addition of
nanoparticles reveals the following heat exchange
enhancement mechanism: (a) Because nanoparticles
have higher thermal conductivity, a higher concentration
of nanoparticles results in a more obvious heat conduc-
tion enhancement. (b) Nanoparticle collisions with the
base fluid molecules and the wall of the heat exchanger
strengthen energy transmission. (c) The nanofluid
increases friction between the fluid and the heat exchan-
ger wall, and thus improves heat exchange capacity. On
the above factors that influence the heat exchange

Figure 9 Reynolds number of Al
2
O
3
/water nanofluid at various
temperatures and concentrations under different mass flow
rates.
Figure 10 Enhanced heat exchange ratio for Al
2
O
3
/water
nanofluid for different concentrations and temperatures at
0.040 kg/s.
Figure 11 Enhanced heat exchange ratio for Al
2
O
3
/water
nanofluid for different concentrations and temperatures at
0.035 kg/s.
Teng et al. Nanoscale Research Letters 2011, 6:488
/>Page 8 of 11
capacity, the collision of these nanoparticles strengthens
the movement of nanoparticles suspended in fluid due
to higher temperature and the increased mass flow rate
of fluid. Furthermore, the higher temperature and mass
flow rate strength en the collision of nanoparticles with
the wall of heat exchanger. These effects influence the
functions of the heat exchanger.

Figures 10, 11, and 12 reve al that ER
he
decreases with
increasing temperature of nanofluid at different mass
flow rates, but the concentration of nanofluid increases
with increased ER
he
. These results show that the
enhanced ratio of heat exchange decreases at higher
temperature. This seems to contradict the statement
above that a high temperature increases the probability
of collision between nanoparticles and liquid molecules,
which can increase heat exchange. This contradictory
phenomenon is main ly because the heat exchanger used
in this study is a rectangular tube with a great cross-sec-
tion aspect ratio (W/H = 17 .3/1.9). The flow rate distri-
bution of the fluid with higher viscosity is relatively
uneven in such cross-section. Thus, the effective cross-
sectional area of the pipe for heat exchange was
decreased to decrease the enhanced ratio of heat
exchange. Figure 8 shows that the decrease rate of visc-
osity of the nanofluid is lower than water at higher tem-
peratures. This means that the viscosity of water is
much lower than nanofluid at high temperatures. This
strengthens the phenomenon of uneven flow rate distri-
bution in the r ectangular tube with a great aspect ratio,
whichinturnenhancestheratioofheatexchangeat
higher temperatures lowe r than the lower temperature
for nanofluid. The maximum enhanced ratio of heat
exchange was obtained at 30°C and 1.5 wt.% for various

mass flow rates, and are about 33-39% compared with
water. In addition, under various experimental condi-
tions, a higher concentration of nanofluid led to an
enhanced ratio of heat exchange increases.
Figures 13, 14, and 15 show the effects of different
nanofluid concentration, inlet temperature, and mass
flow rates on the enhanced ratio of pressure drop
(ER
dp
). In general, the viscosity of nanofluid increases
with increasing nanoparticle loading in the base liquid,
and has a higher friction factor. The pressure drop
experiment in this study s hows a higher concentration
of nanofluid for a higher enhanced ratio of pressure
drop at different temperatures and mass flow rates.
However, there is not a significant trend between the
enhanced ratio of pressure drop and either the flow rate
or temperature. In the whole range of experimental
parameters, the largest enhanced ratio of pressure drop
was 5.6%, occurring at the temperature of 30°C, mass
flow rate of 0.035 kg/s and the concentration of 1.5 wt.
%. The experiments on heat exchange and pressure
drop show that the overall benefits significantly decrease
when nanofluid is used in air-cooled heat exchanger at
higher temperature. The enhanced ratio of pressure
drop becomes even higher than the enhanced ratio of
heat exchange under some conditions, which leads to an
ove rall efficiency of cooling syste m using nanofluid that
is lower than that using water. This is primarily because
the enhanced ratio of heat exchange is lower at higher

temperature. Therefore, the air-cooled heat exchanger
Figure 12 Enhanced heat exchange ratio for Al
2
O
3
/water
nanofluid for different concentrations and temperatures at
0.030 kg/s.
Figure 13 Enhanced pressure drop ratio for Al
2
O
3
/water
nanofluid for different concentrations and temperatures at
0.040 kg/s.
Teng et al. Nanoscale Research Letters 2011, 6:488
/>Page 9 of 11
operating at 30-40°C has the best overall efficiency in
this study.
Conclusions
This study analyzes the characteristics of Al
2
O
3
/water
nanofluid to determine the feasibility of its application
in an air-cooled heat exchanger under laminar flow.
Results confirm that Al
2
O

3
/water nanofluid offers a
higher heat exchange capacity than water, and a higher
concentration of nanoparticles provides an even greater
enhancement ratio of the heat exchange. At higher tem-
perature, however, the nanofluid does not provide
great er enh anced ratio of the heat exchange due to rec-
tangular tube with a large cross-section aspect ratio and
enhanced viscosity ratio. In the whole range of experi-
mental parameters in this study, the maximum
enhanced ratio of heat exchange and pressure drop was
approximately 39% and 5.6%, respectively. The air-
cooled heat exchanger operating at 30-40°C had the
best overall efficiency. Therefore, the temperature and
mass flow rate of the working fluid can affect the
enhanced ratio of heat exchange and pressure drop of
nanofluid in addition to the nanoparticle concentration.
The cross-section aspect ratio of the tube in the heat
exchanger is also an important factor to be taken into
consideration.
Acknowledgements
The authors would like to thank National Science Council of the Republic of
China (Taiwan) and National Taiwan Normal University for their financial
support to this research under contract no.: NSC-99-2221-E-003-008- and
NTNU-99091008, respectively.
Author details
1
Department of Industrial Education, National Taiwan Normal University, No.
162, Section 1, He-ping East Road, Da-an District, Taipei City 10610, Taiwan,
Republic of China

2
Department of Mechatronic Technology, National Taiwan
Normal University, No. 162, Section 1, He-ping Eeast Road, Da-an District,
Taipei City 10610, Taiwan, Republic of China
Authors’ contributions
TPT, YHH, and TCT designed the experiment. TPT and YHH fabricated the
sample. TPT, YHH, and JHC carried out the measurements. TPT, YHH, TCT,
and JHC analyzed the measurements. TPT, YHH, and TCT wrote the paper.
All authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Received: 27 April 2011 Accepted: 9 August 2011
Published: 9 August 2011
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doi:10.1186/1556-276X-6-488
Cite this article as: Teng et al.: Performance evaluation on an air-cooled
heat exchanger for alumina nanofluid under laminar flow. Nanoscale
Research Letters 2011 6:488.
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